The present invention relates to a lockup control system of an automatic transmission with a so-called lockup torque converter, capable of engaging or disengaging a lockup clutch depending on engine/vehicle operating conditions.
In recent years, there have been proposed and developed various technologies for controlling an automatic transmission torque-converter lockup clutch, to provide various merits, that is, reduced internal slippage, improved fuel economy, reduced torque shock during engagement of the lockup clutch, or a better engine braking effect during coasting of the vehicle. One such lockup control system has been disclosed in Japanese Patent Provisional Publication No. 1-206160 (hereinafter is referred to as xe2x80x9cJP1-206160xe2x80x9d) corresponding to U.S. Pat. No. 4,940,122 issued on Jul. 10, 1990. According to a transmission torque-converter lockup clutch slip control system disclosed in JP1-206160, a predetermined speed difference between a driving member (i.e., a pump impeller) and a driven member (i.e., a turbine runner), in other words, a predetermined slippage rate of the transmission torque-converter lockup clutch is permitted by way of closed-loop feedback control, often called xe2x80x9cslip lockup controlxe2x80x9d, so as to effectively reduce torque shock occurring during lockup-clutch engagement. JP1-206160 also teaches the creation of an engine braking force during coasting by way of the slip lockup control. The slip lockup control action executed during coasting is often called xe2x80x9ccoast slip lock-upxe2x80x9d. As is generally known, in an electronically-controlled engine employing a deceleration fuel cutoff device, capable of executing fuel cutoff operation, such as when in overrun (going down a hill) or in city traffic, as well as for engine speed limitation, the fuel cutoff operation usually terminates when the engine speed reduces to below a predetermined engine speed. A fuel cutoff time interval, during which the fuel cutoff operation is continuously executed, can be lengthened by engagement of the lockup clutch during the coast slip lockup control, because of torque transferred from rotating drive wheels back to the engine crankshaft via the lockup clutch partially engaged when the vehicle coasts. This contributes to improved fuel economy.
However, in the torque-converter lockup clutch slip control system as described in JP1-206160, a coast slip lockup area is determined or set irrespective of engine/vehicle operating conditions detected just before a transition to the coast slip lockup area. Concretely, a threshold value required to switch to the coast slip lockup area is set or fixed to a certain vehicle speed that there is a less torque shock, in other words, a less change in rotational inertia of each rotating element even when initiating a coast slip lockup operating mode that permits a slippage between driving and driven members responsively to a rise in the vehicle speed during coasting. Therefore, it is difficult to provide an adequate coast slip lockup area. The inadequate coast slip lockup area results in a poor engine braking performance, for example, a degraded engine braking force, and an increase in fuel consumption. For instance, when the engine/vehicle operating condition, detected just before the transition to the slip lockup area, is the drive operating mode (the vehicle""s driving condition) with the lockup clutch deactivated or released, a lockup piston tends to be attracted toward a torque converter cover by way of action of hydraulic oil in a so-called release chamber defined between the torque converter cover and the lockup piston. When initiating the slip lockup control during such a vehicle""s driving condition, the lockup piston can be easily moved in its axial direction by a slight change in hydraulic pressure, for the same vehicle speed as the vehicle""s coasting condition. Additionally, during the vehicle""s driving condition, there is a less speed difference between engine speed Ne and turbine speed Nt, and thus there is a less torque shock when initiating the slip lockup control. On the contrary, when the engine/vehicle operating condition, detected just before the transition to the coast slip lockup area, is the vehicle""s coasting condition with the lockup clutch deactivated or released, a flow rate of hydraulic oil in an apply chamber of the torque converter side tends to be greater than that in the release chamber. As a result of this, the lockup piston tends to be attracted toward the turbine runner. Under this condition, the distance of the lockup piston relative to the torque converter cover becomes a maximum. For the reasons discussed above, when initiating the coast lockup control during such a vehicle""s coasting condition, the lockup control system has to greatly increase a rate of change in hydraulic pressure with respect to time. As a matter of course, the increased time rate of change in hydraulic pressure may cause a rapid axial movement of the lockup piston. For the same vehicle speed as the vehicle""s driving condition, under the vehicle""s coasting condition the speed difference between engine speed Ne and turbine speed Nt tends to become relatively greater. Therefore, when initiating the coast slip lockup control from a so-called non-lockup state (a fully-disengaged state of the lockup clutch) under the coasting condition during which torque is transferred from rotating drive wheels back to the engine, there is an increased tendency for a comparatively great torque shock to occur owing to both the increased time rate of change in hydraulic pressure and relatively greater speed difference between driving and driven members.
Accordingly, it is an object of the invention to provide a lockup control system of an automatic transmission, capable of enlarging a slip lockup area over wider range of engine/vehicle operating conditions (containing a low and middle vehicle speed range as well as a high vehicle speed range) without undesirable torque shock, occurring due to engagement of a lockup clutch, and thus improving fuel economy while assuring a better engine braking performance.
In order to accomplish the aforementioned and other objects of the present invention, a lockup control system of an automatic transmission with a torque converter having a lockup clutch, comprises an operating condition detector that detects an operating condition on an engine and an automotive vehicle, the operating condition including at least a throttle opening and a vehicle speed, a controller that controls a transmission ratio and an engaging state of the lockup clutch, responsively to an operating point determined based on the throttle opening and the vehicle speed, the controller pre-storing a predetermined lockup control map including at least a predetermined coast slip lockup area, within which the lockup control system executes a slip lockup control mode under a vehicle""s coasting condition that torque is transferred from drive wheels to the engine, so that a speed difference between input and output speeds of the torque converter is brought closer to a predetermined value, the controller determining whether a first transition from a vehicle""s driving condition under which torque is transferred from the engine to the drive wheels to the predetermined coast slip lockup area occurs in a release mode of the lockup clutch or a second transition from the vehicle""s coasting condition to the predetermined coast slip lockup area occurs in the release mode of the lockup clutch, and the controller controlling the lockup clutch depending on whether the first transition occurs or the second transition occurs, so that the lockup clutch is conditioned in the slip lockup control mode when the first transition occurs, and that the lockup clutch is conditioned in the release mode when the second transition occurs.